46o TRANSFORMATION OF ENERGY 



LECTURE XXXVI 

 HELIOTROPISM 



The distribution of light also induces the plant to take up a certain orien- 

 tation in space ; one may speak of phototropism just as we have of geotropism, 

 or — ^since in nature the sun is the only source of light which need betaken account 

 of — we may term it more particularly heliotropism. There are many analogies 

 between geotropism and heliotropism, of which the following are the chief : — 



1. Just as geotropic curvature arises only from the unilateral influence of 

 gravity, so heliotropic curvatures are induced only by unilateral incidence of 

 Bght. The same apparatus which withdraws the plant from the unilateral 

 action of gravity, the klinostat, enables us also to prevent heliotropic curva- 

 tures by rotating the plant in its normal position round a vertical axis. The 

 same effect may naturally be attained by revolving the light round the plant 

 or if we permit the light to stream on the plant on all sides equally (e. g. diffuse 

 daylight). 



2. The different organs of the plant react in different ways to unilateral 

 illurnination. Orihotropic organs arrange themselves parallel with the path of 

 the incident ray, and grow either in a positively hehotropic manner towards 

 the source of light or negatively away from it. Plagiotropic organs, on the other 

 hand, arrange themselves so as to make a definite angle with the incident rays. 

 Organs which are orthotropic as regards gravity are almost always orthotropic 

 as regards light, and the plagiotropic behaviour of other organs shows itself 

 also in heliotropism just as in geotropism. For the most part each organ 

 possesses a power of appreciation of the stimulus of light and of gravity, and 

 if the reactions to these two forces do not annul each other, a positively geo- 

 tropic organ must be negatively heliotropic and vice versa ; that this is the 

 case for the most part experience teaches us. 



From the analogies which exist between them the conclusion has been 

 drawn that geotropism and heliotropism are closely related phenomena, and 

 hence they have frequently been considered in conjunction. The reason for 

 treating them separately here is that, as will be shown later, there are great 

 differences between them. 



As we have noted above, in nature light and gravity act for the most part 

 simultaneously on the plant, and we must assume without deeper knowledge 

 that the one interferes with and not infrequently neutralizes the other. If,, 

 however, both affect the plant in the same way, then one could not learn 

 anything as to the one component which is of interest to us at the present 

 moment, viz. heliotropism, unless it were possible to exclude geotropism alto- 

 gether. This, however, can be done, for we may cause the plant to rotate in 

 the vertical plane on a klinostat and arrange it so that it is illuminated 

 on one side only by placing the axis of the klinostat parallel with a window. 

 Experience has shown that so complicated an experiment is by no means es- 

 sential, since the effect of light often makes itself so prominent that geotropism 

 would appear to be absent altogether. In the present lecture we will confine 

 ourselves to the establishment of this fact — one of the highest importance — 

 and consider the combined action of light and gravity later (Lecture XXXVII). 



The fundamental phenomenon of heliotropism may be seen in any plant 

 which has been cultivated in a room at a distance from the window. Let 

 us consider a seedling of Sinapis alba growing in a culture solution (Fig. 145). 

 It wiU be seen that in a very short time the stem bends towards the window until 

 its axis is approximately parallel with the line of incidence of the light ; it is 

 positively hehotropic and orthotropic. The root behaves in the converse 

 way, it is orthotropic but negatively heliotropic. The leaves on the other 



